1. Field of the Invention:
This invention relates to video signal processing circuitry and particularly relates to circuitry for controlling the brightness of a video signal so that detail of interest in a video picture will appear natural and have good contrast.
2. Brief Description of the Prior Art:
Natural illumination can have an extremely wide brightness range, and will necessarily have a vast range of contrast scales. The human eye adapts itself remarkably well for viewing naturally-lit objects and can with ease perceive detail in shadows and in brightly lit areas as well. Nevertheless, color video cameras and color video display apparatus are not easily adaptable to conditions of natural illumination, and current videocasting practices require special techniques, such as supplemental fill-in lighting, to provide a pleasing yet natural picture.
However, when such special techniques are unavailable, such as during on-scene news reporting, the picture presented on a display apparatus can be harsh and unpleasant. For example, if an on-the-spot newscast takes place at night with a newscaster at the news scene standing in front of a bright source, such as a flashing neon sign, the picture is likely to be harsh and without good detail. In such a scene, the presentation of the neon light is bright but the other objects in the picture are dark, and the contrast range among such objects is extremely narrow. Thus, except for the neon sign, the picture appears objectionably dim and observation of detail in the picture is difficult.
This problem can be understood by considering that while a color camera can be responsive to input light having an illumination range of from several hundred to several hundred thousand lux, the electrical output of the camera is limited to a range of, for example, 1 volt peak-to-peak. The input light must have a limited illumination range, e.g. 100 to 200 lux or several thousand to several tens of thousands of lux, in order that all of the video output signal remain within the range of 1 volt peak-to-peak. If these illumination limits are not observed, a conventional color television camera and display apparatus will not provide a good, pleasing picture.
Brightness adjustment in the video transmission is now carried out to a limited extent by use of so-called gamma (.gamma.) correction. This process compensates for the differences in gamma values between the image pickup tube of a television camera and the cathode ray tube (CRT) of a television receiver.
Normally, the picked-up image is gamma-corrected before transmission so that the net gamma value of the image pickup and image display will be unity.
Conventionally, gamma correction is carried out on the image pickup side so that the output signal is skewed logarithmically at the saturated (white) side of the brightness range. Then, the skewed curve is expanded somewhat at the CRT, due to its inherent gamma characteristic, so that the picture brightness is correct.
Generally, if the overall gamma characteristic is logarithmic, the dark picture portions will have expanded contrast, and fine dark or shadow detail is reproduced. Conversely, if the gamma characteristic is exponential, the bright portions will have expanded contrast, and detail in brightly lit areas will be clear.
Further, the lower illumination intensity portions of the video signal are affected by noise in the video apparatus. Consequently, a good video picture cannot be obtained for any scene unless the picture brightness is properly adjusted to span the entire dynamic range of the video apparatus. Accordingly, the actual brightness of an object in the scene does not convert exactly to a particular level of the video output signal, especially if the object is not evenly illuminated. The image of such an object in an unevenly-lit scene is not easily visible when reproduced on a video screen, and hence fatigues the eyes, making viewing somewhat tiring and unpleasant.